Mixed-valence molybdenum oxide as a recyclable sorbent for silver removal and recovery from wastewater

Silver ions in wastewater streams are a major pollutant and a threat to human health. Given the increasing demand and relative scarcity of silver, these streams could be a lucrative source to extract metallic silver. Wastewater is a complex mixture of many different metal salts, and developing recyclable sorbents with high specificity towards silver ions remains a major challenge. Here we report that molybdenum oxide (MoOx) adsorbent with mixed-valence (Mo(V) and Mo(VI)) demonstrates high selectivity (distribution coefficient of 6437.40 mL g−1) for Ag+ and an uptake capacity of 2605.91 mg g−1. Our experimental results and density functional theory calculations illustrate the mechanism behind Ag+ adsorption and reduction. Our results show that Mo(V) species reduce Ag+ to metallic Ag, which decreases the energy barrier for subsequent Ag+ reductions, accounting for the high uptake of Ag+ from wastewater. Due to its high selectivity, MoOx favorably adsorbs Ag+ even in the presence of interfering ions. High selective recovery of Ag+ from wastewater (recovery efficiency = 97.9%) further supports the practical applications of the sorbent. Finally, MoOx can be recycled following silver recovery while maintaining a recovery efficiency of 97.1% after five cycles. The method is expected to provide a viable strategy to recover silver from wastewater.

Introduction and Experimental sections. The main findings with important opinions are acceptable. The mathematical terms need to be added. The authors need to consider these points in the revision stage.
Response: Many thanks for this great suggestion. The abstract was revised in detail. The excellent properties of amorphous MoOx have been highlighted and more mathematical terms have been added in the abstract. Moreover, the self-reinforcing reduction mechanism of MoOx for Ag + recovery and the novel regeneration strategy of MoOx were briefly elucidated. More details were added in the revised manuscripts.
"Achieving the sustainable circulation of adsorbents in Ag + recovery from wastewater with adsorption technology is desirable but challenging. Herein, a wastefree molybdenum oxide (MoOx) adsorbent with mixed-valence (Mo(V) and Mo(VI)) was designed for selective Ag + recovery from complex wastewater. MoOx exhibits a remarkable uptake capacity (as high as 2605.91 mg g -1 ) and an ultrahigh selectivity (distribution coefficient up to 6437.40 mL g -1 ) for Ag + . Even in the presence of different interfering media, the removal efficiency (RE) of MoOx for Ag + can be maintained above 94%. Density functional theory (DFT) calculations and experimental characterizations unveiled that this exceptional Ag + uptake is due to a self-enhancing reduction mechanism. Specifically, as an electron donor on MoOx, Mo(V) trends to reduce Ag + to metallic Ag, which decreases the energy barrier for subsequent Ag + reductions. Impressively, MoOx can realize selective Ag + recovery (RE=97.9%) from actual Ag + -containing wastewater. In addition, the raw material (MoO4 2-) can be recycled to re-synthesize MoOx after capturing Ag, realizing closed-loop circulation of adsorbents. Through 5 cycles, the regenerated MoOx can still maintain over 97.1% Ag + uptake performance. The disruptive adsorbent regeneration strategy with waste-free characteristics reported in this work will be helpful to realize the persistent utilization of adsorbents." Please see Page 2 Lines 21-38. Comment 4. References: Many references are not adjacent to this study. The authors need to take notes in the revision stage and cite relevant references including highimpact journals to make the manuscript to a broad range of readers.
Response: Thanks very much for this suggestion. We have carried out a meticulous examination of the article references. References that were inconsistent with this paper were deleted and replaced. In the part about mechanism explanation, we cite references from high-impact journals to enhance the convincing power of the article.
The relevant modifications were in the revised manuscript and marked in red. Moreover, the authors need to cite high-impact articles to make the manuscript highlevel. The following specific articles may take be noted in the revision stage of Chemical Engineering Journal, 266 (2015) 368-375;Microchemical Journal, 154 (2020) 104585;Chemical Engineering Journal, 307 (2017) 85-94;Chemical Engineering Journal, 324 (2017) 130-139;Journal of Environmental Chemical Engineering, 7 (2019) 103087;Chemical Engineering Journal, 320 (2017) 427-435;Journal of Molecular Liquids, 284 (2019) 502-510;Chemical Engineering Journal, 288 (2016) 368-376;Journal of Environmental Chemical Engineering, 7 (2019) 103378;Composites Part B: Engineering, 171 (2019) 294-301;Journal of Molecular Liquids, 298 (2020) 112035;Chemical Engineering Journal, 259 (2015) 611-619. Response: Many thanks for Reviewer's constructive comment. As you mentioned, a variety of functional materials have been designed to remove and recover different heavy metal ions in recent years. We consider the amorphous MoOx adsorbent to be progressive compared to some current composite adsorbents based primarily on the following points: (1) MoOx is synthesized in one step by electrochemical method, which is simple and fast. Also, the preparation procedure employs water and hence is nontoxic.
(2) The recovery of silver ions on MoOx is based on a self-enhancing reduction mechanism. This enables MoOx has an ultra-high Ag + adsorption capacity.
Moreover, the self-enhancing mechanism also endows MoOx with excellent selective adsorption properties, enabling the targeted recovery of Ag + from complex wastewater.
(3) Targeting MoOx, a closed-loop strategy of synthesis-adsorption-dissolutionregeneration is proposed for the first time in this work, which provides a new concept for the persistent utilization of adsorbents. The related discussions were added to the revised manuscript and marked in red.
According to your comments, the relevant references have also been cited in the introduction to enhance the level of this paper.
"Much effort has been dedicated to developing adsorbents with high capacity and excellent selectivity [12][13][14][15]  Response: Many thanks for the Reviewer's kind concern. We recognized this shortcoming and carefully revised the results and discussion sections. A more in-depth analysis and discussion has been added to the existing experimental results. The mechanism of silver ion recovery by MoOx was systematically elucidated. In addition, we have cited the references you mentioned above to confirm the certainty of our work.
Specific modifications were marked in red in the revised manuscript.
"High adsorbent selectivity is essential for recovering heavy metal ions from actual wastewater 40 ." Please See Page 6 Lines 132-133 "In addition, the pH value decreases after Ag + adsorption (e.g., from 6.0 to 3.8), Response: Many thanks for your suggestion. For ion selectivity, we constructed binary and multiple heavy metal ion coexistence systems under laboratory conditions to assess the selection of MoOx for silver ion adsorption. As shown in Figure 1b, the Ag + uptake efficiency is 49.66-473.33 times that of other metal ions at the equal initial concentration in the binary mixed solution. The distribution coefficients (kd) of MoOx for Ag + is 6437.40 mL g -1 , which is 4 × 10 4 to 2 × 10 5 times more than for other metal ions (only 0.03-0.15 mL g -1 ), proving that the amorphous MoOx possesses an excellent selectivity towards Ag + adsorption. Besides, the flow-through recovery tests are conducted to further investigate the Ag + capture performance from actual Ag +containing electroplating wastewater. In Figure 4a, the concentration of Ag + after filtration was 0.42 mg L -1 , yet, the other competing metal ions shows no obvious concentration change. This results demonstrate that MoOx maintains excellent Ag + selectivity even in actual wastewater.  Response: Many thanks for the Reviewer's great advice and the section of discussion has been rewritten. In the revised discussion, we highlighted the new strategy of MoOx closed-looped Ag + recovery, briefly described the innovation of the self-enhanced reduction mechanism for MoOx capture Ag + , and summarized the excellent adsorption performance as well as the strong anti-interference of MoOx.
Besides, MoOx is considered to be one of the best adsorbents for Ag + capture by comparison with other works. Finally, the application of MoOx and the close-looped recovery strategies in the field of precious metal recovery was prospected. More details were added in the revised Manuscript and marked in red.

"Discussion
In summary, we proposed a strategy for the closed- The manuscript is recommended to be published after revision based on the comments.
Response: Many thanks to the Reviewer for the very positive comments and valuable suggestions, which are much helpful to improve the scientific merits of the manuscript.

Comment 1. Adsorption behavior of other monovalent cations should be mentioned.
Response: Many thanks for the Reviewer's comment. Based on your suggestion, two binary solution (composed of Ag + /Na + , Ag + /K + and Ag + /Li + ) and a multi-metal solution consisted of the above four metal ions was used to evaluate the effect of coexisting monovalent cations on Ag + adsorption, respectively. The concentrations of all the metal ions were 20 mg L -1 . A piece of amorphous MoOx was soaked into 100 mL of the above solutions for 10 h under evenly stirred condition at room temperature (25 ± 2 °C), respectively. Then, the solution was filtered through a syringe filter, and the filtrates were analyzed by an atomic absorption spectrometer (AAS, ContrAA 700, Analytik Jena, Germany) to determine the concentrations of Ag + , Na + , K + and Li + .
As shown in the figure below, MoOx almost exclusively extracts Ag (above 99.5%) but not the other three monovalent cations (below 0.2%) either in binary or multiple coexistence systems, indicating that the presence of monovalent metal cations does not interfere to the adsorption of Ag + on MoOx. This is in line with our expectations.
Because, Ag + have a higher reduction potential and are more easily reduced to metallic Ag on MoOx, compared with other monovalent metal cations. Attributed to the unique reduction recovery mechanism of MoOx, it can overcome the interference of monovalent metal cations and selectively reduce and recover Ag + from wastewater.

Comment 3. Influence of anions on the Ag(I) adsorption behavior should be referred.
Response: Many thanks to you for this great suggestion. In order to investigated the influence of different anions on Ag + uptake, two binary solution (composed of Ag + /Cr 6+ and Ag + /Sb 5+ ) and a multi-metal solution consisted of the above three metal ions was prepared, respectively. The concentrations of all the metal ions were 20 mg L -1 . A piece of amorphous MoOx was soaked into 100 mL of the above solutions for 10 h under evenly stirred condition at room temperature (25 ± 2 °C), respectively. Then, the solution was filtered through a syringe filter, and the filtrates were analyzed by an atomic absorption spectrometer (AAS, ContrAA 700, Analytik Jena, Germany) to determine the concentrations of Ag + , Cr 6+ and Sb 5+ .
From Figure R2, the presence of heavy metal oxygenated anions almost no effect on the capture of Ag + . MoOx achieves over 98.5% removal efficiency for Ag + , both in binary and multi-metal co-existence systems. Furthermore, the effect of different concentrations of NO3on the capture of Ag + was also evaluated. As seen in Figure   has been widely applied to model the isotherm data associated with the reduction/oxidation removal/recovery of As 3+ , Cr 3+ , Au 3+ , Hg 2+ , and Ag + .
The capture of silver ions by MoOx is considered as a process of adsorption followed by reductive deposition. In the first stage of Ag + extraction, silver ions are uniformly adsorbed onto the MoOx surface, which is a monolayer adsorption, in accordance with the Langmuir model. Immediately afterwards, the adsorbed Ag + are reduced on MoOx and the original adsorption sites are released to capture more silver ions. Moreover, the deposited metallic Ag can effectively reduce the energy barrier for subsequent Ag + deposition, making subsequent Ag + reduction easier, which is the reason for the ultra-high Ag + adsorption capacity of MoOx. The relevant details of the modifications were added to the revised manuscript and marked in red.
"The adsorption isotherm (Figure 1a) of the amorphous MoOx demonstrates that the Ag + uptake capacity is increased promptly with increasing equilibrium concentrations (initial concentrations ranging from 10 mg L -1 to 250 mg L -1 ). For a comparison, bare FTO achieves scarcely any Ag + removal within 120 min ( Figure S7).
Two typical isothermal adsorption models (Langmuir and Freundlich models) were used to fit the above data (Table S2), and the adsorption process fits better with the Langmuir model (R 2 = 0.948). Then, the Langmuir model was employed to predict the theoretical capacity of MoOx to remove Ag + , and the model has been widely applied to the isotherm data associated with the reduction/oxidation removal/recovery of As 3+ , Cr 3+ , Au 3+ , Hg 2+ , and Ag + 35-39 . To our delight, the maximum adsorption capacity (qm) was calculated to be as high as 2605.91 mg g -1 , implying that amorphous MoOx possesses an outstanding application potential in the field of Ag + recovery." Please see  Response: Many thanks for Reviewer's great suggestion. The purity of the recovered Ag + is calculated based on the change in concentration of all metal ions in the wastewater. We believe that in the material regeneration stage, ammonia will not only dissolve the MoOx adsorbent, but also dissolve the impurity ions adsorbed on MoOx (Cu 2+ , Zn 2+ , Ni 2+ , and Co 2+ ), so as to obtain higher purity metallic Ag. On this basis, we detected the concentration of heavy metal ions in the regeneration solution and recalculated the purity of the recovered Ag. 50 mL 0.2 mol L -1 of ammonia solution was used to dissolve MoOx and impurity metal ions. As seen in Figure R3, trace amounts of Ag + and most of impurity ions are dissolved into the ammonia solution.
This result confirms that the purity of the recovered Ag has been improved. Combined with the data in Figure 4a, the purity of the recovery Ag was recalculated to be as high as 99.79%, which has a high recovery value. The section of the manuscript relating to the purity of the recovered Ag has been revised accordingly and marked in red. The details for calculating the purity of recovered Ag + were listed in the Supplementary   Information (Text S5). Figure R3. The concentration of Ag + , Cu 2+ , Ni 2+ , Zn 2+ and Co 2+ in the NH3 H2O solution after the dissolution of MoOx.

Text S5
The purity of the recovered metallic Ag can be calculated by the following formula: Where the p is the purity of recovered Ag (%), m1 is the mass of recovered Ag (mg), and m is the total mass of recovered metals (including Ag + , Cu 2+ , Ni 2+ , Zn 2+ , and Co 2+ , mg).
The values of m1 and m are obtained from the difference in the concentration of heavy metal ions in the wastewater and regenerative solutions, where the volume of the wastewater and regenerative solutions is 100 mL and 50 mL, respectively. Through calculation, the values of m1 and m are 1.955 and 1.959 mg, respectively. Therefore, the purity of silver recovered from wastewater is as high as 99.79%.

Comment 6. Reproducibility of the MoOx production can be shown not only by the Ag(I) recovery ratio but also by the ratio of Mo(V) to Mo(VI)?
Response: Many thanks for the Reviewer's valuable comment. Following your suggestion, the high-resolution Mo 3d XPS spectra after 6 regenerations were analysed to demonstrate the superiority of closed-looped regeneration of MoOx ( Figure R4).
Impressively, the ratio of Mo(V) to Mo(VI) on MoOx after 6 regenerations is 2.1, which is similar to the ratio on the original MoOx (2.5). Therefore, MoOx can still maintain the original chemical structure and excellent Ag + capture performance after 6 regenerations, which proves that it has a good reproducibility. Figure R4. High-resolution XPS spectra of Mo 3d orbitals for MoOx and MoOx after 5 cycles. effectively reduces the energy required for the reduction of Ag + , which caused more silver ions to be reduced. This results well explain why the amount of deposited Ag is higher than the amount of Mo(V) oxidized on MoOx. In addition, the in-situ EPR spectra indicate the production of hydroxyl radicals during Ag + deposition, verified the reliability of the DFT calculation results. Therefore, it can be conclude that the recovery mechanism of Ag + on MoOx is mainly a self-enhancing reductive deposition. The detailed discussions were in the revised manuscript (Pages 9-11).

Comment 2. L65. The state-of-the-art is incomplete and does not include the possibility for incinerating metal-loaded sorbent.
Response: Many thanks for your kind reminder. We have revised the manuscript accordingly and marked it in red.
"The former method involves selectively adsorbing Ag + by constructing specific cavities that match Ag + , while the latter method utilizes the very strong ability of sulfur to bind Ag + , which can be attributed to the Lewis soft-soft interactions. Although these materials exhibit excellent selectivity for Ag + adsorption, the following shortcomings remain: (i) To achieve the recovery of Ag + , these adsorbents need to elute Ag + through desorbents (such as acid, alkali or organic solution) or incinerate the adsorbents after adsorption. However, the eluents used cause complicated post-processing procedures and may lead to secondary pollution. Incineration is problematic as the process consumes much energy and generates waste gas." Please see Page 3 Lines 66-70 Comment 3. L82-L92: study outcomes should not be listed in the introduction section.
Response: Many thanks for your kind concern. We have removed the statement about the study outcomes and revised the introduction. The details were added in the revised manuscript and marked in red.
"Herein, we successfully designed and synthesized an amorphous MoOx with reductive Mo(V) based on redox precipitation mechanism using an electrochemical technique. Then, batch Ag + -recovery experiments were performed to evaluate the performance of amorphous MoOx. Through experimental analysis and density functional theory (DFT) calculations, we also fundamentally elucidated the mechanisms of MoOx capture Ag + . Moreover, a flow-through reactor was designed to evaluate the Ag recovery and demonstrate the superior application potential of MoOx to recover metallic Ag from actual Ag + -containing wastewater. In addition, a closedloop recycling method to recover Ag + and regenerate MoOx was tested. Finally, we evaluated the regeneration performance of MoOx and considered the economic benefits for MoOx recovery Ag to further demonstrate the potential for practical application."

Please see Page 4 Lines 87-97
Comment 4. Figure 1: too small, recommended to increase the quality of figures.
Response: Thank you very much for your suggestion. We have made adjustments to Figure 1 and the resolution of all images in this paper has been improved.  Supplementary Information (Figure S17). Figure S17. Economic analysis of Ag + recovery from 1 t Ag + -containing wastewater. The authors are not serious about revision. The authors need to seriously revise the manuscript to get acceptance from this high-quality journal. A major revision is required.
The authors investigated Ag recovery from wastewater using molybdenum oxide adsorbent to safeguard public health. The authors designed the work systematic way by performing some valuable experimental works accordingly. It is also necessary to critically evaluate new data and not make hasty conclusions that may lead to misinterpretations. However, several points are important to be addressed before going to possible publication in this high-quality journal. Also, the authors need to address all points in the revision stage for a broad range of readers' understanding.
-The English language needs to check carefully in the revision stage because of many careless mistakes in many positions.
-The Figure's quality needs to be improved in the revision stage.
-Abstract: The abstract section is completely different from the Introduction and Experimental sections. The main findings with important opinions are acceptable. The mathematical terms need to be added. The authors need to consider these points in the revision stage.
-References: Many references are not adjacent to this study. The authors need to take notes in the revision stage and cite relevant references including high-impact journals to make the manuscript to a broad range of readers.
-Introduction: There are many studies reported in the literature regarding diverse metal removal and recovery based on different functionalized materials. Composite materials are growing attention for diverse pollutants removal based on their specific functionality. Based on this, do the authors think that the present molybdenum oxide adsorbent is an improvement when compared to other composite materials? The authors need to indicate such points for a broad range of readers. Moreover, the authors need to cite high-impact articles to make the manuscript high-level. The following specific articles may take be noted in the revision stage of Chemical Engineering Journal, 266 (2015) (2021) 116667.
-The ion selectivity study needs to be judged as the wastewater containing diverse metal ions.
-The elution/regeneration study needs to be evaluated for the potentiality of the molybdenum oxide adsorbent as a cost-effective.
-Conclusion also needs to be rewritten. Include the following: new concepts and innovations demonstrated in this study, a summary of findings, a comparison with findings by other workers, and a concluding remark.
I would like to see the revised manuscript.
Reviewer #2 (Remarks to the Author): The manuscript was appropritely revised based on my questions and comments to Authors. I do not have any additional comments to the revised manuscript.
Reviewer #3 (Remarks to the Author): 1. General comments: Efforts of the authors to improve the discussion are appreciated and lead to a better interconnection of the findings with existing scientific concepts. Certain explanations however reflect a limited understanding of fundamental chemical processes and could have been refined at a more detailed level.
In particular, this relates the description of reaction enthalpy and activation energy.
While the material and application mechanism can definitely be subject for further studying, it is perceived as sufficiently documented in the proposed manuscript.

Unresolved issues:
-L205: "because" -Figure S1: The oxidation state notation of Mo6+ is invalid and should be Mo(VI) as the element evidently does not prevail as a hexavalent cation.
-L222 and L254: The rate determining step and, respectively, binding energy are still abbreviated in the text, while this is uncommon. Response: Many thanks to the Reviewers for the recognition of this work.

Reviewer #3 (Remarks to the Author):
General comments: Efforts of the authors to improve the discussion are appreciated and lead to a better interconnection of the findings with existing scientific concepts.
Certain explanations however reflect a limited understanding of fundamental chemical processes and could have been refined at a more detailed level. In particular, this relates the description of reaction enthalpy and activation energy.
While the material and application mechanism can definitely be subject for further studying, it is perceived as sufficiently documented in the proposed manuscript.

Response:
We appreciate the Reviewer for the positive comments on this work, and we also thank you for pointing out the shortcomings in our previous round of responses. Based on your suggestions, we reinterpreted the self-reinforcing reduction mechanism of Ag + on MoOx.
According to the XRD, SEM and XPS characterizations of MoOx after captured reduces the energy barrier for the reduction of Ag + , which caused more silver ions to be reduced. The in-situ EPR spectra indicate the production of hydroxyl radicals during Ag + deposition, verifying the reliability of the DFT calculation results. Therefore, it can be conclude that the recovery mechanism of Ag + on MoOx is mainly a self-enhancing reductive deposition.
This work investigates the performance, mechanism, and application of MoOx to capture silver ions. However, the recovery other precious metal ions (such as gold, platinum and palladium) on MoOx has yet to be verified and studied. In addition, molybdenum oxide materials possess great photoelectric properties. The combination of photo/electrochemistry with MoOx for the recovery of precious metals also deserves further study.

Comment 1. L205: "because"
Response: Many thanks for the Reviewer's kind concern. We fixed this spelling error in revised manuscript and marked in red.
"This occurs because incipient Ag nanoparticles possess better conductivity (Nyquist plots shown in Figure S10), which could act as an "e-bridge" for transferring electrons from MoOx to reduce more outer Ag + , and thus, the strip structure was formed 47 ." Please see Page 8 Line 193 Comment 2. Figure S1: The oxidation state notation of Mo 6+ is invalid and should be Mo(VI) as the element evidently does not prevail as a hexavalent cation.
Response: Many thanks for the Reviewer's kind reminder. We recognized this error and the oxidation state notation in Figure S1 has been corrected. Comment 3. L222 and L254: The rate determining step and, respectively, binding energy are still abbreviated in the text, while this is uncommon.

Supplementary
Response: Many thanks to Reviewer for the kind reminder. We carefully examined the manuscript and modified all these abbreviations to their full names. The modifications were marked in red in the revised manuscript.
"Notably, after Ag + uptake, the peak position of Mo(V) shifted to lower binding energies by almost 0.9 eV (i. e., from 234.96 eV to 234.08 eV for Mo(V) 3d3/2). This is owing to the interfacial electron transfer between Ag + and Mo(V) species, which could induce the partial structural evolution of external Mo-O 50, 51 , resulting in a larger binding energy shift." "Moreover, the rate-determining step on MoOx is the transformation of *O→*OOH with a free energy of 2.317 eV, but the rate-determining step on MoOx-Ag changed from *O→*OOH to *OOH→O2." Please see Pages 8-9 Lines 213-234